1. Field of the Invention
The present invention relates to an optical communications module for use in optical communications which is provided with at least one laser diode, at least one photodiode, or both for transmitting and receiving optical signals, and particularly to an optical communications module in which electrical crosstalk between the signal-transmitting section and the signal-receiving section, between the signal-transmitting sections, and between the signal-receiving sections is reduced. The present invention also relates to a method for producing the optical communications module.
2. Description of the Background Art
In the optical communications system using light for transmitting information, the optical communications modules described below are known for transmitting and receiving optical signals propagating over an optical transmission line such as an optical fiber transmission line.    (a) Ryuta Takahashi et al. have proposed the following optical module in the report entitled “Packaging of optical semiconductor chips for SFF (small form factor) optical transceiver” included in the proceedings of the 1999 Electronics Society Conference of IEICE (The Institute of Electronics, Information and Communication Engineers of Japan) (page 133, number C-3-28). The optical module incorporates an Si substrate provided with two V-shaped grooves and metallized patterns (electrode patterns). A laser diode (hereinafter referred to as an LD) for signal transmission and a photodiode (hereinafter referred to as a PD) for signal reception are placed on the metallized patterns. Optical fibers are securely held in the V-shaped grooves to be butt-connected with the LD and PD for enabling the interchange of the signal light.
The module is illustrated in FIGS. 9 and 10. FIG. 9 is a plan view and FIG. 10 a longitudinal cross section. An insulating layer 3 (SiO2) is formed on the rear half of a rectangular Si substrate 2. An LD 4 for signal transmission and a PD 5 for signal reception are mounted on the insulating layer 3. Two parallel V-shaped grooves 7 and 8 are formed on the front half 6 of the Si substrate 2. The V-shaped grooves are formed with high precision by the anisotropic etching of an Si single crystal. Optical fibers 9 and 10 are fitted in the V-shaped grooves 7 and 8 and fixed there. The end of the optical fiber 9 faces the LD 4 and that of the optical fiber 10 faces the PD 5. As shown in FIG. 10, a light-emitting layer 27 of the LD 4 is positioned at the same level as that of the center of the optical fiber 9. Light emitted from the side face of the LD 4 propagates over the optical fiber 9 to the outside. Conversely, light from the outside optical fiber propagates over the optical fiber 10 to reach the side face of the PD 5. Therefore, the PD 5 must be a side-illuminated type. Front- and rear-illuminated types cannot be used.
The LD 4 and PD 5 are placed with left-right symmetry with respect to the centerline of the Si substrate 2. Consequently, the optical fibers are parallel to each other, and the LD 4 and PD 5 are parallel to each other. This configuration can reduce the space needed. Because the LD 4 and PD 5 are mounted on the same Si substrate 2 in parallel, the module can reduce the parts prices, mounting cost, and size.
The Si substrate 2 has a rectangular shape with a length of 5 mm and a width of 2.5 mm. Actually, the optical fibers, LD 4, PD 5, and other components are covered with a transparent resin. Furthermore, the entire unit is encapsulated with a shape-holding resin to form a module of a plastic-molded package type.    (b) The published Japanese patent application Tokukaihei 11-68705 entitled “bidirectional optical WDM (wavelength division multiplexing) tranceiver module” has proposed a tranceiver module. In the tranceiver module, an SiO2 insulating layer is formed on an Si substrate. A Y-shaped optical waveguide made of GeO2-doped SiO2 enclosed by SiO2 is formed on the insulating layer. A PD is placed at the position corresponding to the bottom end of the letter Y An optical fiber is connected to the position corresponding to the left upper end of the letter Y. An LD is placed at the position corresponding to the right upper end of the letter Y. A WDM filter that selectively reflects 1.3-μm light and selectively transmits 1.55-μm light is placed at the junction point of the letter Y. The 1.3-μm light for signal transmission emitted from the LD propagates over the optical waveguide, is reflected by the filter, enters the optical fiber, and transmits to the outside.
Conversely, light having propagated over a optical fiber from the outside enters the optical waveguide, passes through the filter, and enters the PD to give signals. The PD, also, is a side-illuminated type. Front- and rear-illuminated types cannot be used. In this tranceiver module, the LD and PD are placed with front and rear symmetry, not with left-right symmetry.
Both tranceiver modules explained in (a) and (b) above have a structure in which an LD and a PD are mounted on an Si substrate, and they are connected to the outside optical fiber through an optical transmission medium such as an optical waveguide or fiber. The propagation direction of light, the chip faces, and the substrate surface are parallel to one another. Consequently, light propagates two-dimensionally without requiring a wide space. As a result, a tiny, low-cost optical tranceiver module can be produced.
However, the tiny, low-cost optical tranceiver module newly makes a problem of electrical crosstalk caused by the intrusion of noise from the signal-transmitting section to the signal-receiving section. There are two causes for the crosstalk. FIG. 11 is a lateral cross section at a plane including the LD and PD in the optical tranceiver module shown in FIGS. 9 and 10. The causes of the crosstalk are explained as follows by using FIG. 11.    (1) The insulating layer 3 (SiO2 layer) sandwiched between the metallized patterns 20 and 24 and the Si substrate 2 is a dielectric material and has capacitances C2 and C3.    (2) The Si substrate 2 is a semiconductor substrate having finite resistances R4, R5, R6, and R7.
Therefore, the LD 4 and the PD 5 are connected to each other through an AC circuit composed of the resistances R4, R5, R6, and R7 in the Si substrate 2 and the equivalent capacitances C2 and C3 in the insulating layer 3. Because the insulating layer 3 is thin, the capacitances C2 and C3 are high. Because the frequency is high, the reactances 1/jωC2 and 1/jωC3 are low. The Si substrate 2 is made of an n- or p-type Si single crystal having low resistivity for a semiconductor. Consequently, the value of R4+R5+R6+R7 is considerably low. As a result, the total impedance Z=1/jωC2+1/jωC3+R4+R5+R6+R7 is considerably low. This low impedance allows intense electrical signals to be fed to the LD to leak into the PD and mix with received signals. Thus, electrical crosstalk is caused between the LD and PD through the Si substrate.    (c) Sonomi Ishii et al. have proposed an idea to reduce the crosstalk between the signal-transmitting section and the signal-receiving section in the report entitled “Crosstalk analysis of MT-RJ optical subassembly” included in the proceedings of the 2000 Electronics Society Conference of IEICE (page 352, number SC-3-7). The idea employs a shield plate inserted into a groove provided at a location between the signal-transmitting section and the signal-receiving section on an Si substrate. The groove has a depth less than the thickness of the Si substrate. The distance between the optical axes of the two sections is 750 μm. It can be said that the shield plate is provided to prevent crosstalk caused by electromagnetic waves rather than by electric current. To block electromagnetic waves intruding from the LD to the PD, the shield plate is substantially high from the surface of the Si substrate. However, because the impedance Z=1/jωC2+1/jωC3+R4+R5+R6+R7 is low, the electrical crosstalk caused by an AC current flowing through the Si substrate remains high.    (d) The published Japanese patent application Tokukai 2002-170984 (filed as Tokugan 2000-367925) entitled “Optical communications unit” has proposed a structure to reduce electromagnetic crosstalk. In the proposed structure, a series of through holes are provided somewhere between the LD and PD on the Si substrate. A metallic shield plate having a comb-shaped foot is inserted into the holes. The foot is soldered to a metallic base plate attached to the back face of the Si substrate. The LD and PD are isolated from each other with a metallic box incorporating the metallic shield plate as a separator. The structure is intended to completely suppress crosstalk caused by electromagnetic waves. Electric signals for the LD emit substantially intense electromagnetic waves because they are composed of high-speed pulses. The electromagnetic coupling between the LD and PD placed in proximity to each other allows the electromagnetic waves to intrude into the PD as noises. To prevent the intrusion, the prior art (d) has devised a structure in which the electromagnetic waves are contained in the metallic box that separately encases the LD and PD sections.
There are three types of crosstalk between the signal-transmitting section (LD section) and the signal-receiving section (PD section): optical crosstalk, electromagnetic crosstalk, and electrical crosstalk. Because the signals have high frequency, electromagnetic crosstalk is noticeable. However, attention must be paid to the presence of electrical crosstalk. In the situation under consideration, the Si substrate 2 is a good conductor rather than a semiconductor. The insulating layer 3 is thin and allows AC current to flow. As a result, electrical crosstalk becomes significant. The prior arts (a) and (b) described above have no effective measure against the problem of the crosstalk between the signal-transmitting and -receiving sections. The prior arts (c) and (d) merely describe measures against electromagnetic crosstalk. It cannot be said that sufficient attention is paid to the electrical crosstalk in the prior arts cited above.
The fact that the Si substrate is a good conductor in this case may have been neglected in the above-described prior arts. If the substrate is produced by using an insulating material such as a ceramic material, the problem of the electrical crosstalk may be solved. However, a ceramic substrate is devoid of the advantages the Si substrate innately has. The Si substrate is advantageous in that photolithographic techniques form with extremely high precision the V-shaped grooves, the positioning marks for the LD and PD, and the various patterns, and substrates can be mass-produced using a large wafer. Now, Si single crystals are produced by the most advanced single-crystal production technology. Large Si wafers (300 mm in diameter) are available at low prices. The production technology of Si wafers has been highly sophisticated to such an extent that the formation of the V-shaped grooves and marking can be easily performed with high precision. The Si substrate has attractive features that cannot be obtained with a ceramic substrate.